Method for the production of low sulfur sponge iron



Patented Jan. 24, 1950 UNITED STATES PATENT TOFFICE METHOD FOR THE PRODUCTION OF LOW SULFUR SPONGE IRON No Drawing. A lication October 7, 1947, Serial No. 778.452

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 1 Claim.

The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to us of any royalty thereon in accordance with the provisions of the act of April 30, 1928 (Ch. .60, 45 Stat. L. 467).

This invention relates to the production of easily reducible metals from their ores, and more particularly to the production of such metals having a low sulfur content. Still more particu' larly this invention relates to the production of metallic iron and similar metals of the iron group of the periodic system wherein thefinal product has a low sulfur content.

In processes for the production of various easily reducible metals of a low sulfur content, particularly rotary kiln sponge iron processes, varsulfurizing agents. Such processes have been deious alkaline earth compounds have been employed and suggested as applicable for use as described and extensively referred to in' the Bureau of Mines Bulletin 2'70 (1927) U. S. Patent 2,380,406 (Buehl), and other well known literature.

The Buehl patent discloses the use of a precalcined mixture of coal and either lime, dolomite, or other alkaline earth compounds ground to pass a lll-mesh screen for the production of low sulfur sponge iron. This method involves the calcining of the coal and the sulfur control agent and this operation adds to the cost of the process.

Accordingly, itis an object of this, invention to provide a method for the production of low sulfur content metals from their ores,'which is inexpensive and does not involve any special processes which add appreciably to the cost.

ance with this invention include the processes for the reduction of ores of metals with coke or other carbonaceous reducing agents at elevated temperatures; Examples of such process are those for the production of metallic iron, copper,

cobalt, nickel, cadmium, and other metals which,

- 2 can be reduced from their oxides, carbonates, and the like, with carbonaceous reducing agents such as charcoal, coals, petroleum, coke, bitumen,

bituminous coals having coking characteristics, and the like. In general processes for the treatment of ores of the metals of the iron group in periodic system can best be treated in accordance with this invention, and of these, processes for the production of sponge iron in a rotary kiln constitute preferred members.

In a typical embodiment of the preferred rotary kiln process for the production of the sponge iron a mixture of the raw materials in granular form is fed into the elevated end of the kiln, which is fired axially from the lower or discharge end. In the reducing zone, which is approximately one third the length of the kiln measured from the discharge end, a temperature of 1800 to 2000 F. is maintained. The reduced material is discharged through a cooling chamber, which is an integral part of the kiln and which prevents the material from being reoxidized by air during cooling. This discharge is then passed through a magnetic separator, which removes the magnetic sponge iron from the non-magnetic char and desulfurizing agent.

A study was made of the form in which sulfur exists in sponge iron and of the factors that influence the transfer of sulfur into the sponge iron when solid reducingagents are employed. It was indicated that if no sulfur-controlling agents were employed, sulfur contents as high as 0.25

I per cent could be expected. The addition of lime,

limestone, or dolomite in powdered form to the feed did not lower the sulfur content suiiiciently to make the sponge iron desirable as a melting stock. In contrast to these results, the addition of sized dolomite to the feed reduced the sulfur content to the range of 0.02 to 0.05 per cent. Sized lime, while beneficial, was not nearly as effective as sized dolomite, sulfur contents being in the range of 0.04 'to0.0'l per cent. This difference in the results is attributed to the tendency of the particles of lime to be pulverized by tumbling and the abrasive action of the charge in the kiln. Limestone is converted to lime at the operating temperature and therefore loses its physical strength. a In contrast, dolomite retains its physical strength when heated, therefore, the particles do not crumble in the rotating kiln. As a result ofthe pulverizing action in the rotary kiln, fine particles of basic desulfurizing agent adhere to the surface of the sponge-iron granules, and this surface coating has been found to contain most of the sulfur in the sponge-iron product.

An accurate sulfur balance is difficult to obtain because of sulfur loss in the combustion gases. An approximate sulfur distribution is given below for a charge consisting of equal parts of low sulfur magnetite ore (0.02 per cent S) and coke (0.8 :per cent S) with the addition of sized dolomite to the extent of percent of the weight of the ore. Sponge iron with a sulfur contentof u 0.03 per cent was produced inan internallyfired rotary kiln from these raw materials. In .this event, low-sulfur oil (0.3 per cent S) was used to supply the heat for the-process. With these raw materials, over 90 per cent of the sulfur in the charge is contained inithe reducing agent. About 2.5 per cent of the sulfur in the charge appears in the sponge iron, 60 to '70 per cent of the-initial sulfur remains in the unconsumed coal andcoal ash rejected by the magnetic separator, '16 per cent is contained in the dolomite, and the remainder passes oif to the combustion gases. When no desulfurizin g agent is employed, the sponge iron containsfrom 16 to 20 per cent of the sulfur in the charge.

.A partial summary of experimentalevidence to demonstrate the existence of. a basic coating containing most of the sulfur in the sponge-iron concentrate is presented in the following paragraphs, and will show how the invention may be carried out, but is not limited thereto.

Example 'Sponge iron was produced in an externally heated, horizontally rotating alloyretort 8 inches in diameter and inches long. Samples were prepared using minus 8-;mesh unsized dolomite, K111111518 plus 100-mesh dolomite and a control wherein no desulfurizing-agent was used.

Cont7'ol.(a) Sponge-iron was preparedfrom amixture of 2 kg. of Chateaugay ore and 1.5 kg. of No. 4 anthracite (0.7 per centsulfur) withno .desulfurizing agent employed. The 'spongeiron so produced contained 0.07 per ,cent'sulfur before .sizingand the sulfur appears to be uniformly distributed through the sponge iron particles.

Unsized dolomite.--(b) Sponge iron was produced from the same mixture of iron ore and .coal, except that 0.3 kg. of minus 8-mesh unsized dolomite was added as 'the desulfurizing agent. The dolomite had the normal size distribution that results from grinding, and contained 42 per I -:ucts so produced are :tabulated below in part'as the table.

Tamer-Percent sulfur in sponge iron fractions Sponge iron partlclesize range (mesh) -+4s' '--48+65 -c5+100 -1no+200 '1 1N0 dolomite 0.01 "0.07 0. 07 0.06 --8..mesh dolomite. DLOG ..O..10 0.16 0.57

#2049100 mesh dolo- :mlto .(Lllld 703017 0.02;; 0:06

If a surface layer containing a high sulfur content is present on the particles, finer sized particles will have a much higher sulfur content than the coarse particles. It is evident from the results of the table that the use of minus 8-mesh dolomite, including the fine 1d11st,..results in a surface coating (small particles have higher sulfur .content than large particles) whereas the use of sized dolomitejrom which the minus 100-mesh ;par.ticles.have been removed does not result in the formation of a surface layer with an appreciable sulfur content on the sponge iron at 1950 F. and is therefore an effective desulfurizing agent.

From a comparison of the data in parts (b) and '(c) of the example, it appears that the elimination of the minus 100-mesh fraction of the dolomite prevents .the formation of a surface coating and :results in a low-sulfur sponge iron.

As shown in the foregoing descriptions and ex- .ample, iron of a very loW sulfurcontent can be produced by heating .iron ore and a suitable car- .bonaceous reducing agent withsized dolomite as .a desulfurizing agent. It has ,been found that employing unsizeddolomite as the desulfurizing agent in such a process results in a much higher sulfur content .in the finished iron product.

While the invention has .beenparticularly described in connection with ironproduction by the sponge method its utility .is not limited thereto, since in other processes the reduction of iron ore occurs in a similar manner. The use of properly sizeddolomite as avdesulfurizing agent in a blast furnace should result in the production of low sulfur iron in the upper portion of the furnace beforecarburization and meltingoccur and therefore the metal would not require .desulfurization with slags in the lower section of the furnace as occurs in modern practice. With proper design and operation of blast furnaces,.such production .oflow sulfur iron before melting occurs would aid considerably in the manufacture of .low sulfur Di iron.

While the invention has been particularly described inrelation to the reduction ofiron ores, it islikewise usefulin the reduction of other metallio ores by means of coke, where it is desirable to have a low sulfur content in the finished metal.

Since many widely diiferingembodiments of the invention will occur to one skilled in the art, the invention is not limited to the specific details illustrated and described, and various changes can be made therein without departing from the "spirit and scope thereof.

What is claimed is:

In a process for the production of sponge iron, the steps which comprise forming a reducible mixture of iron ore and a carbonaceous reducing agent, adding thereto dolomite which has been sized to about minus 20 plus mesh, and re- 'ducing said mixture.

RUSSELL C. BUEHL. JOHN P. RIOTT. EARLE P. SHOUB.

REFERENCES CITED The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS .Number Name Date 1,065,890 .Sieurin n June24, 1913 2,380,406 .Buehl July 31, 1945 OTHER REFERENCES -fProduction of Low Sulphur SDDnge .Iron," Metals Technology, October 1946 (TP 2093). 

